1. Field of the Invention
The field of the invention is wireless communications and more particularly, is receivers utilized for signal reception in a wireless communication system.
2. Background
Wireless systems are becoming a fundamental mode of telecommunication in modern society. In response to increasing market demand, the existing analog system, Advance Mobile Phone Service (AMPS), is being augmented with systems based upon digital modulation schemes, such as code division multiple access (CDMA), the Global System for Mobile (GSM), and 3rd Generation (3G) follow-on protocols. As deployment of these systems is not uniform either with respect to frequency band or geographic coverage, there is a need for a receiver that is capable of handling communication over more than one of these standardized protocols or bands so as to provide connectivity when geographically roaming. In order to achieve this, it is desirable to have a receiver that is capable of receiving signals which have been modulated according to several different modulation techniques and or frequency bands, such as, providing CDMA or GSM connectivity for both the Cellular and Digital Frequency Bands and also providing AMPS connectivity.
Most existing receivers are implemented using double conversion receiver architectures. A double conversion receiver architecture is characterized in that the received RF signal is converted to a real IF signal and this IF signal is subsequently converted to baseband. However, these double conversion receivers have the disadvantage of utilizing a great number of circuit components such as narrowband IF filters, thereby increasing the cost, size and power consumption of the receiver. In addition, gain control is also typically applied at the IF which adds complexity and desensitizes the receiver.
A direct conversion receiver provides an alternative to the traditional double down conversion architecture. Direct conversion is characterized in that the received signal is converted directly from the radio frequency at which it is received to baseband. One such technique was disclosed by Williams in U.S. Pat. No. 5,557,642 entitled “Direct Conversion Receiver For Multiple Protocols.” In direct conversion, an antenna receives RF signals that have been digitally modulated according to a predetermined standard. The output of the antenna is passed to a low noise amplifier (LNA). The LNA amplifies the incoming signal. The output of the LNA is coupled to an automatic gain control (AGC) and filtering block. An automatic gain control and filtering block controls the magnitude and spectral content of the received signal. The output of the automatic gain control and filtering block is coupled to an amplifier that further amplifies the signal. The output from the amplifier is input into a sample and hold circuit. The sample and hold circuit is clocked by a first clock having a frequency f1. The output of the sample and hold circuit comprises a series of copies of the modulated signal centered about multiples of the clock frequency f1. The output of the sample and hold circuit is coupled to an oversampling delta-sigma converter. The delta-sigma converter receives a second clock having a frequency, f2, which is an integer multiple of the frequency f1. In this way, the delta-sigma converter loop oversamples the output signal provided by the sample and hold circuit. Thus, after decimation filtering, it provides a quantized representation of the modulated signal.
The inclusion of the sample and hold circuit, however, generates unneeded multiple spectral harmonic replicas of the signal since only one is utilized, which both add to the power and increase interference. In addition, the construction of the sample and hold circuit requires the use of high frequency circuit elements and design techniques to minimize the effects of sampling jitter and the resultant undesirable modulation of the signal even when the subsampling frequency is relatively low. For example, if a 2 GHz carrier signal is subsampled with a modest 200 MHz clock, the aperture time during which the sample and hold circuit samples the signal must be on the order of several picoseconds in order to avoid significant distortion and “droop” of the sampled signal. The sample and hold circuit is typically implemented using some combination of diodes, FET switches or operational amplifiers that typically only operate sufficiently linearly over a small portion of their overall functional voltage range. In addition, the use of subsampling reduces the oversampling ratio that would be achieved by sampling at the carrier frequency or higher thereby significantly reducing the dynamic range of the delta-sigma converter loop. For example, the resolution of a delta-sigma converter is dependent upon the oversampling ratio. First, second, third, and fourth order delta-sigma converters optimally achieve 1.5, 2.5, 3.5, and 4.5 bits of resolution per octave of oversampling ratio, respectively. For example, using 200 MHz sampling clock, the Williams' architecture sacrifices 4.98 bits of resolution (30 decibels (dB)), 8.30 bits of resolution (50 dB), and 11.63 bits of resolution (70 dB), for first, second, and third order delta-sigma converters, respectively, as compared to sampling at the carrier frequency.
Recognizing that in a typical system application with a dynamic range requirement of 90 dB or greater, the dynamic range over which the input signal varies is larger than the dynamic range over which subsequent elements, such as the sample and hold circuit and delta-sigma loop, can operate, Williams inserted the AGC and filter circuit before the sample and hold circuit. The filter itself is typically implemented using of discrete analog components, which further increases the size and cost of the receiver. Finally, the inclusion of automatic gain control creates a DC offset error that is a function of the automatic gain control.
A similar prior art approach to implementing a RF communications receiver is described in Shen et al. U.S. Pat. No. 5,640,698. In Shen's method, the RF signal is band limited by a RF filter after which it is amplified and further noise filtered to avoid aliasing the LNA noise into the signal channel. The output of the noise filter is then inputted into a sample and hold circuit whose sampling rate is selected so that it sub-samples the RF signal in such a manner that the signal band is translated to a discrete-time image frequency. This discrete-time image frequency is successively further down sampled, anti-alias filtered and amplified to yield a low frequency discrete-time signal containing a down-converted channel of frequencies that contain the frequency of interest. This resultant low frequency discrete-time signal is then digitized in an analog to digital converter, filtered, and demodulated to reveal its baseband information. Salient features of the sub-sampling approach are an extensive use of narrowband analog RF, IF, or lowpass filters; multiple stages of analog conversion and amplification that limit dynamic range to that of traditional multiple down-conversion receivers; a low sampling rate; use of an approximate or exact sub-harmonic of signal carrier; a single ADC at IF that is sufficiently high to avoid images; and potential problems with sample and hold jitter.
An alternate approach to direct conversion was proposed in a co-pending patent application, U.S. patent application Ser. No. 09/339,063, by Wes Masenten and Ron Hickling, titled “DIRECT CONVERSION SIGMA-DELTA RECEIVER” and filed on Jun. 23, 1999. This alternative radio receiver directly converts a RF modulated carrier signal to a digital representation of the modulating signal. The receiver concurrently translates the RF modulated carrier signal to baseband using a commutator and digitizes the translated signal. The resultant receiver replaces the traditional double down conversion receiver and its associated analog filters with a direct down-conversion implementation.
Hence, the prior art receivers, such as traditional double down conversion receivers, have limitations that affect their performance. For example, the prior art receivers often suffer from a narrow dynamic range, thus requiring the inclusion of separate circuitry to cover multiple bands, automatic gain control circuits, and off-substrate filtering. Therefore, there is a need for an improved receiver.